Burner

ABSTRACT

The invention relates to a burner for gaseous, fluid or powdery fuels, into which three components are introduced: a fuel ( 40 ); an oxidizing gas ( 10 ), for example air; and an inert gas ( 20 ), for example gases produced by combustion, nitrogen or water vapor. Two components, for example, air and inert gas, are mixed together and propelled by at least one injection stage ( 95 ) arranged at different positions in relation to the movement of the fuel.

TECHNICAL FIELD

The present invention relates to a burner for gaseous or gasifiablefuels. According to an important aspect, the burner of the inventionallows the combustion at moderate temperature, without visible flame,and is characterized by a particularly low emission rate, in particularas regards the CO, solid particles, and oxides of nitrogen (NO_(x)). Thepresent invention is suitable for use with a large number of fuels.

STATE OF THE ART

Several variants of burners are known in the art, making it possible touse any kind of fuel. Flame burners cause a localized high temperature,which results in the formation of unwanted pollutants, such as NO_(x).

Flameless burners are also known which combine good energy efficiencywith low emissions. In these burners, also known as FLOX® burners, aportion of the flue gas is recycled in the combustion zone. In this way,the mixture is depleted and a diffuse combustion is obtained whosetemperature is more uniform than in conventional devices. Theseadvantages, however, require proper adjustment of the combustionparameters. In these burners, the fuel and the oxidizing gas areinjected at the same time and at the same place into the combustionchamber so that an optimal and homogeneous concentration of thesecomponents is ideally obtained. If optimal conditions are not achieved,imbalances may occur which result in imperfect combustion.

In order to achieve the optimum combustion conditions, the moreefficient known burners are complex constructions, requiring precisemanufacture with a large number of parts assembled with closetolerances. This obviously has a detrimental effect on the cost of theproduct and sometimes also on its reliability.

The burner of the invention can be adapted to use a large number ofcombustible substances, for example combustible gas, flammable liquids,solids, or also gases and volatile products resulting from a pyrolysisreaction. In this latter application, the burner of the invention isparticularly efficient and can be advantageously coupled to a pyrolysisfurnace to produce charcoal and energy from wood or plant products. Whencharcoal is used for soil improvement or sequestered in any way, thesefacilities can produce renewable energy with a positive CO₂ balance.

Document EP2184538 discloses a FLOX burner with recirculation of fluegas.

Document EP2669575 discloses a burner in which the oxidant flow isperiodically varied. However, this document does not describe steppedinjectors with respect to fuel movement. Similarly, EP1486729 disclosesa burner with air and water injectors arranged laterally, but withoutrecycling of flue gas. Moreover, this burner is optimized for thethermal degradation of PCBs, and operates at very high temperatures.DE19619919, DE19613777, DE102004034211, EP0413104 and U.S. Pat. No.6,155,818 describe other solutions of interest as technologicalbackground.

BRIEF SUMMARY OF THE INVENTION

One aim of the present invention is to provide a burner free from thelimitations of known devices.

Another aim of the invention is to provide a burner that allows cleancombustion, that is energy efficient, and that is accompanied by a lowlevel of NO_(x) and CO emissions.

According to the invention, these aims are achieved in particular bymeans of a burner with recirculation of inert gas or combustion gas inthe combustion zone, and staged injection, defined by the appendedclaims.

BRIEF DESCRIPTION OF THE FIGURES

Examples of implementation of the invention are indicated in thedescription illustrated by the appended figures in which:

FIG. 1 schematically illustrates a burner according to one aspect of theinvention associated with a fuel source. FIG. 1a illustrates theenlarged detail ‘A’ therefrom.

FIGS. 2, 3, 4 and 5 illustrate variants of the burner of the invention.

EXAMPLE(S) OF EMBODIMENT OF THE INVENTION

In the embodiment of the invention illustrated by FIGS. 1 and 1 a, thecombustion chamber 70 is a vertical cylindrical body 70 which may bemade of steel, or any other suitable material. Of course, the inventioncould be implemented by combustion chambers of different shape and/ororientation: these characteristics are not essential for the invention.

In the illustrated example, a fuel gas 40 is produced by a pyrolysisfurnace 45 in which wood, or a similar material, is converted intocharcoal with the production of combustible gases and vapors. The fuels,produced and preheated by the pyrolysis, enter through a lower openingand move upwardly by natural convection, pass through the combustionzone 77 in which they are converted into burnt product 60, and exitthrough the upper opening.

The present device has been specifically designed for burning pyrolysisproducts, and has provided very satisfactory results in thisapplication, being capable of high energy efficiency with low emissions.However, the invention may be adapted to burn any fuel, be it gaseous,volatile, an atomized liquid, or a sprayed solid substance. The movementof the fuel can be by natural convention, as is the case in thisexample, or be pushed by a blower, for example.

The duct 80 serves to introduce combustion air into the combustionvolume 77, which is essential for combustion. In the illustratedexample, it descends into the combustion zone while remaining in theperipheral position. This is not an indispensable feature of theinvention. The duct 80 could indeed be in a central position, or alsoexternal to the combustion chamber 77. The air 10 is drawn by a fan 13or by any other appropriate means creating a pressure difference. Theinvention also includes variants in which the combustion air is drawninto the combustion chamber by a depression created downstream, forexample by a natural convection or forced convection chimney.

The main part of the oxidizing gas 10 is not discharged directly intothe combustion zone. The duct 80 is surrounded, at least partially, by aconduit 90 of inert gas 20, in this case a flue gas taken downstream ofthe combustion zone 77, therefore a gas with zero oxidizing capacity, orat least greatly reduced oxidizing capacity, compared with that of theoxidizing gas 10. The oxidant 10 is mixed with the inert gas 20 by theinjectors 35, and the mixture 30 is injected through the nozzles 95 intothe combustion zone 77. The speed of ejection of the air 10 by theinjectors 35 generates, by the Venturi effect, a depression which drawsthe inert gas 20 to descend into the pipe 90.

Importantly, the injection of the burnt air/gas mixture 30 ismulti-stage, arranged one after the other in the combustion zone,relative to the movement (vertical in this example) of the fuel, so thata vertical combustion profile is established. In addition, the speed ofthe pulsed air causes a powerful transverse mixing. Technical tests haveshown that this device makes it possible to obtain an optimal andlow-emission combustion of gases 40. Furthermore, adjusting it isexcessively simple because it is reduced to the choice of the speed ofthe fan 13. This parameter is, moreover, not very critical, and thetests performed have shown that wide variations in ventilation velocityhave a very limited effect on the operation of the burner of theinvention, which is therefore particularly suitable for simpleinstallations and does not require sharp monitoring.

The construction of the burner is also particularly simple and does notrequire precision machining and fitting. It is worth mentioning that thegeometry of the injectors 35 could be much more refined than what isshown, as evidenced by the large number of Venturi injectors known inthe art. The illustrated structure, however, has provided quitesatisfactory results and has the advantage of simple and inexpensiveconstruction.

Optionally, a direct injection of air 15 into the combustion chamber canbe provided upstream of the injectors 95 and higher in the combustionzone. This injection makes it possible to burn any combustible productsthat would not have been entirely consumed in the combustion zone 77.

FIG. 2 shows a variant of the burner of the invention wherein the inertgas conduit is disposed within the oxidizing gas pipe. In this variant,the inert gas is pressurized, for example by a pump or a fan, not shown,and mixes with the oxidizing gas (e.g. air) in the injectors 35 whichare placed on the side wall of the conduit 90 opposite the nozzles 95 ofthe air duct 80. This variant has the same advantages as that of FIG. 1with regard to the quality of the combustion, with the difference thatthe inert gas is taken from outside and that it is the driving fluid ofthe injectors 35, so that the air 10 (oxidant) is sucked by the Venturieffect.

Several kinds of inert gases can be employed in this variant, forexample nitrogen, carbon dioxide, or pressurized water vapor. FIG. 2illustrates in particular a particularly advantageous variant in whichthe inert gas 20 is pressurized water vapor produced by a steamgenerator 22 (illustrated here very schematically), preferably heated bythe burner itself.

FIG. 3 refers to another variant of the invention in which the drivingfluid is a pressurized inert gas which draws air (combustion gas) by theVenturi effect. The difference with the examples presented above is thatthe mixture of air and inert gas is not at the level of the stages 95,but in a Venturi unit 35 placed higher. Although the figure shows asingle unit 35, it is quite clear that without exceeding the scope ofthe invention, a plurality of units 35 may be upstream of the injectionstages 95.

Analogously, FIG. 4 shows a variant in which the oxidizing fluid 10 ispulsed air which also serves as a driving fluid in a Venturi unit 35placed upstream of the injection stages 95, in which the air is mixedwith a inert gas 20 produced from combustion. Also in this case, asingle injector is shown to simplify the drawing, but the invention mayinclude several.

The embodiment of FIG. 5 comprises two injection groups 101 and 102,positioned at different heights in the combustion chamber. In theillustrated example, the injection groups 101 and 102 are positionedperipherally and opposite each other, but this is not an essentialfeature. The injection groups 101 and 102 are supplied with combustiongas by two blowers 13, 130, as illustrated, or by a single commonblower, or by natural aspiration, as mentioned above.

In the first injection group 101, the inert gas conduit 90 rises to theupper regions of the combustion zone, for example above the injectionnozzles 195 of the second injection group, or substantially to the sameheight of said nozzles, so as to suck inert gas at maximum temperature.

The second injection group has the function, among others, to mitigatethe combustion temperature in the upper regions of the burner. For thispurpose, the external pipe 120 is not connected with the hot flue gases,but with a source of cold and inert gas, for example water vapor, or thecombustion gas, but cooled. In the example illustrated, the externalpipe is in communication with the combustion gases downstream of a heatexchanger 160. It is also possible to combine the two types of gas,either to recycle combustion gases, cooled or not, into the externalpipe, and to inject vaporized or nebulized water into it.

The burner of FIG. 5 makes it possible to obtain an optimal verticaltemperature profile and favorable to a complete and clean combustion.The first injection group 101 recycles into the combustion zone very hotgases, taken near the hottest point of the chamber, which allowscomplete combustion, and also makes it possible to consumelow-volatility fuels, for example fuel oil.

Experience has shown, however, that a burner with a single injectorgroup can reach, at the top, temperatures that are too high and harmfulto the materials, and giving rise to the production of NO_(R). Thisdisadvantage is overcome by the second injector group 102, whichrecycles in the combustion zone gases at a lower temperature than thoserecycled by the first group. For example, the first injector group couldrecycle flue gases having a temperature of 800° C., and the second group102 could recycle still flue gases, cooled to 200-300° C.

The invention presented is not limited to the examples described withthe aid of the figures. In particular, it also includes variants inwhich a mixture comprising a combustible gas and an inert or oxidizinggas is injected in a staged fashion in the claimed manner. In otherembodiments, the conduits 80 and 90, which are shown here as coaxial,could be simply parallel, with passages in correspondence of theinjection stages, without one completely surrounding the other. Theseconduits are furthermore not necessarily straight, as shown. Othervariations are possible, and the invention includes all those having thefeatures defined by the appended claims.

REFERENCE NUMBERS USED IN THE FIGURES

-   10 Influx of oxidizing gas-   13 Fan-   15 Direct injection of oxidizing gas-   20 Influx of inert gas or flue gas-   22 Steam generator-   30 Gas mixture-   35 Injector/mixer-   40 Influx of fuel-   45 Fuel source, e.g. pyrolysis oven-   60 Evacuation of combustion products-   70 Wall, combustion chamber-   77 Combustion zone-   80 Oxidizing gas conduit-   85 Conduit of depleted mixture-   90 Inert gas conduit-   95 Injection nozzles-   101 First injection group-   102 Second injection group-   120 Cold gas-   130 Source of inert gas-   140 Mixed gas mixture-   160 Heat exchanger-   195 Injection nozzles of the second group

1. A burner comprising a combustion zone into which three components areintroduced by a first injection group: i. a fuel; ii. an oxidizing gas,for example air; iii. an inert gas, for example gases from combustion,nitrogen, or water vapor; wherein: two components are mixed together andthe resulting mixture is injected and pulsed at a plurality of stagesinto the third component in the combustion zone, and in that the fuel isin motion and traverses the combustion zone, while the oxidizing gas andthe inert gas are mixed together by one or more injectors, so that themixture thus obtained is drawn into the combustion zone at a pluralityof injection stages arranged at different positions relative to themovement of the fuel.
 2. The burner of claim 1, wherein said fuelcomprises one or more of: hydrocarbon gas, mineral oil, solid fuel. 3.The burner of claim 1, wherein said fuel comprises gaseous and/orvolatile products emitted by a pyrolysis furnace.
 4. The burner claim 1,wherein said mixture of two components and the injection of the mixtureinto the third component takes place in an injector based on the Venturieffect.
 5. The burner of claim 1, wherein said oxidizing gas is air andsaid inert gas is a part of the flue gases from the combustion, the airbeing introduced under overpressure into a conduit of the oxidizing gasprovided with a plurality of injectors, wherein the conduit for theoxidizing gas is surrounded at least partially by an inert gas conduit,equipped with injection stages opposite the injectors.
 6. The burner ofclaim 1, in which said oxidizing gas is air and said inert gas ispressurized water vapor or a pressurized inert gas introduced into aninert gas conduit provided with a plurality of injectors surrounded atleast partially by a conduit for the oxidizing gas, equipped withinjection stages opposite the injectors.
 7. The burner of claim 1,wherein said oxidizing gas is air and said inert gas is pressurizedwater vapor or a pressurized inert gas introduced into an inert gasconduit provided with a plurality of injectors surrounded at leastpartially by a conduit for the oxidizing gas, equipped with injectionstages opposite the injectors.
 8. The burner of claim 1, comprising oneor several orifices for the injection of combustion gas downstream ofthe combustion zone.
 9. The burner of claim 1, wherein said inert gas iswater vapor produced by a steam generator.
 10. The burner of claim 1,wherein said inert gas is a part of the flue gases from combustion inthe combustion zone, and said oxidizing gas is air drawn by a fan. 11.The burner of claim 1, comprising a second injection unit, connected toa source of colder inert gas of the inert gas recycled by the firstinjector.
 12. The burner of claim 11, wherein the first injection groupinjects an inert gas consisting of a part of the combustion flue gasestaken from the upper region of the combustion chamber, substantially atthe same level, or higher, than the second injection group.